1. Field of the Invention
The present invention relates to a developing method for developing an electrostatic latent image formed on an electrostatic latent image-bearing member such as an electrophotographic photoreceptor or an electrostatic recording derivative with a two-component developer for visualization in electrophotography.
2. Description of the Related Art
Although a large number of methods have been conventionally known as electrophotography, a general method involves: using a photoconductive substance to form an electrostatic latent image on an electrostatic latent image-bearing member (a photosensitive drum) by using various means; developing the electrostatic latent image with a developer (toner) for visualization; transferring the toner image onto a transfer material such as paper as required; and fixing the toner image onto the transfer material by virtue of heat, pressure, or the like to obtain a copy. Developing methods in electrophotography are mainly classified into a one-component developing method in which a carrier is not needed and a two-component developing method in which toner and a carrier are used. In particular, the two-component developing method is suitably used for a digital combination machine or a full-color copying machine where high image quality is required.
Known as the two-component developing method is the following method. The method involves: forming a magnetic brush of a two-component developer having nonmagnetic toner and a magnetic carrier on a developer bearing member (a developing sleeve) into which a magnet is incorporated; coating the carrier with the magnetic brush with a predetermined thickness by means of a developer layer thickness regulating member; conveying the magnetic brush to a developing region opposed to a photosensitive drum; and bringing the magnetic brush close to/in contact with the surface of the photosensitive drum while applying a predetermined developing bias between the photosensitive drum and the developing sleeve in the developing region to visualize the electrostatic latent image as a toner image.
In recent digital combination machines and full-color copying machines each adopting the two-component developing method, in accordance with improved image quality and increased speed, a so-called multi-stage developing system has been widely proposed, in which development is performed by using multiple developer carries (developing sleeves) In the multi-stage developing system, the number of occasions of friction between a magnetic brush and the surface of a photosensitive drum is large, and a large developing region can be secured. Therefore, a high-definition and high-density image can be obtained, so the system has been preferably used. However, it is difficult to form a uniform developer layer on each of the multiple developing sleeves. In particular, if transportation of a developer between developing sleeves fails, scattering of the developer, uneven coating, or the like is apt to occur. In addition, stress to be applied to the developer between a developing sleeve and a developer layer thickness regulating member (between S and B) or between a developing sleeve and another developing sleeve (between S and S) is large, so deterioration of the developer is apt to occur. In consideration of those problems, it may be important to control the flowability of the developer.
With regard to the flowability of a two-component developer, JP 11-073005 A or JP 11-174731 A proposes a two-component developer characterized in that the two-component developer has an apparent density of 1.2 to 2.0 g/cm3 and a degree of compression of 5 to 19%. Thus, in an image forming method in which a change in bulk density is suppressed and a change in permeability of a two-component developer is detected by using an inductance of a coil to control a toner concentration, the developer has a suppressing effect on fluctuations in image density and tint. However, in the multi-stage developing system, when such a two-component developer having flowability is used, it becomes difficult to form a uniform developer layer on each of the multiple developing sleeves, and, in particular, transportation of the developer between developing sleeves is apt to be nonuniform, so insufficient coating or the like may occur.
In addition, with regard to a multi-stage developing system using a two-component developer, for example, JP 2003-295602 A, JP 2003-323043 A, and JP 2003-323052 A each propose improvements of a magnetic pole structure, stirring means, and the like to form a uniform developer layer on a developing sleeve and to prevent deterioration of a developer in a developing unit. However, an improvement of a developer having flowability suitable for a multi-stage developing system still requires further investigation.